These panels, obtained by mixing the filler with a liquid mixture comprising the phenolic resin, and an agent for hardening this resin, have a certain number of drawbacks in their present method of manufacture, due to their structure and to the fact that their manufacture is inadequately controlled.
One of these drawbacks resides in that under the conditions in which the manufacture is presently carried out, for obtaining a phenolic resin with open cells, on the one hand the latter is subject to infiltrations in particular of water and on the other hand, burns easily owing to the presence of air which is trapped therein. In addition, this resin having open cells lacks cohesion.
It is possible to minimise these drawbacks by placing the panel obtained between facings of a suitable type, for example steel facings, but it is only possible to envisage facings defining a casing around the panel, which casing has its own cohesion owing to the lack of cohesion of the panel itself. In particular, it is not possible to produce facings in the form of a layer of polymerisable resin integral with the panel over the entire surface area. The constraint limits the currently known use of panels based on phenolic resin.
Another drawback of these panels resides in their lack of homogeneity, fillers having a density less than the density of the resin tending to rise when the resin has not been completely polycondensed and in particular at the time of an expansion phase, whereas the denser resin tends to drop. A cut made in a panel produced in the currently known manner reveals a much greater density of filler in the upper part of the panel than in its lower part, the result being great friarbility of the panel in its upper part. It is also apparent that the resin expands in a heterogeneous manner, expanding much more freely towards the top of the panel than at the bottom, which detracts further from the mechanical properties of the panel in its upper part.
Furthermore, difficulties are encountered in working industrially with polystyrene balls and phenolic resins with their currently known composition, owing to their short shelf life, i.e. the short period of time during which they can be used after preparation, which considerably limits the quantity of resin which can be prepared and consequently used for a manufacture.